1. Technical Field
This invention pertains to a process of simulated moving bed (SMB) chromatography. It is particularly directed to operation of a simulated moving bed in coupled relation to a second chromatographic separation process. It provides for the recovery of a betaine and/or invert fraction from sugar solutions and the coupled production of a high purity sucrose product.
2. Background Art
U.S. Pat. No. 4,412,866 describes the operation of an SMB to separate the components of a feed stock. A resin bed is divided into a series of discrete vessels, each of which functions as a zone within a circulation loop. A manifold system connects the vessels and directs in appropriate sequence to (or from) each vessel each of the four media accommodated by the process. Those media are generally referred to as feed stock, eluent, extract and raffinate, respectively. As applied to a sugar factory, a typical feed stock is sucrose solution, the eluent is water, the extract is an aqueous solution of sucrose and the raffinate is an aqueous solution containing nonsucrose, such as salts and high molecular weight compounds. The SMB disclosed by the ""866 patent is of the type sometimes referred to as a xe2x80x9ccontinuous SMBxe2x80x9d to distinguish it from another type, sometimes referred to as axe2x80x9csequential SMB.xe2x80x9d Unless otherwise indicated, the term xe2x80x9cSMBxe2x80x9d is used in this disclosure to denote a continuous SMB.
The largest single loss of sugar values from a typical sugar factory is attributable to molasses formation. Molasses comprises the byproduct (or waste) stream remaining after repeated crystallization procedures are applied to recover purified sugar. This molasses is typically of such low purity that further crystallization procedures for the recovery of additional sugar are economically impractical. SMB arrangements similar to those disclosed by the ""866 patent are used in sugar factories to process molasses, typically producing a product fraction of relatively high (e.g., 90%) purity and low ash content and a byproduct fraction, comprising, typically, 40-50% of the feed, of relatively low purity and low ash content. (As used in the sugar industry, xe2x80x9cpurityxe2x80x9d specifies percent by weight sucrose of the solids contained in a sample, on a dry weight basis.)
In the sugar beet industry, the byproduct fraction contains most of the betaine values of the molasses feed. Betaine, being the most abundant nitrogenous compound found in molasses, has been recognized as a commercially useful byproduct, notably for use in animal feeds. In the sugar cane industry, the byproduct fraction contains most of the invert sugar (i.e., glucose and fructose) values of the molasses feed. The invert is a valuable digestible carbohydrate. As used herein, the term xe2x80x9cinvertxe2x80x9d refers to xe2x80x9cinvert sugarxe2x80x9d (a mixture of glucose and fructose formed in equal quantities by the hydrolysis of sucrose).
In the typical operation of SMB chromatography, the product sucrose fraction (extract) is contaminated to some extent by betaine and/or invert. Such contamination reduces the recovery of these valuable byproducts and reduces the purity of the sucrose product. This disadvantage is attributable to the steps inherent in typical SMB operation. The SMB is initially inventoried with solids to an equilibrium state, and, thereafter, feed and eluent are fed into the continuously recycling inventory while extract and raffinate are withdrawn from the recycling inventory. In the context of this disclosure, the term xe2x80x9cinventoryxe2x80x9d refers to the distribution and identity of chemical species constituting the recycle stream. This recycling of inventory is generally a very favorable aspect of the SMB, because material is subjected to a very long chromatographic path for separation, dependent upon chosen recycle rates and inventory levels. As a result, difficult-to-remove materials, such as certain color compounds, are efficiently removed. In comparison, inventory build-up and long separation path length (due to circulation) are inconsistent with batch chromatography. Eluent use is also much lower in SMB chromatography because the eluent is continuously recycled (eluent phase) as part of the internal inventory.
Unfortunately, providing a continuous, quickly recycling, internal configuration results in only two well-separated components, one on each end of the separation profile. Some nonsucroses, such as ash and high molecular weight compounds, move much more quickly than sucrose through the monovalent form ion exchange separation resins typically employed and, therefore, move to the front of the recirculating separation profile. Some other nonsucroses, such as betaine, invert and certain amino acids, move much more slowly than sucrose through these resins. As a consequence, either the faster moving nonsucroses or the slower moving nonsucroses must always be crossing through the separated sucrose, thereby contaminating the sucrose.
The extract obtained from the chromatography of sucrose solutions is conventionally subjected to crystallization procedures, resulting in an acceptably pure saleable sugar product. The highest present day commercial standards for overall recovery of crystallized sucrose from beet molasses suggest a chromatography efficiency of 92 purity sucrose at 90% recovery. At this purity level, subsequent recovery by crystallization procedures is typically about 87%, with a loss of some sucrose to a second molasses (60 purity). Overall, a xe2x80x9csuperiorxe2x80x9d combined result of conventional chromatography and crystallization procedures has been the recovery of up to about 78% of the sucrose content of the initial molasses as crystalized sucrose.
U.S. Pat. Nos. 4,359,430 and 5,127,957 describe methods for the recovery of a betaine fraction from various sources, such as the molasses produced by a sugar factory. The ""957 patent discloses a discontinuous circulation, batch-wise operation. The method involves shutting off all input and output streams to the SMB while maintaining circulation through the resin bed. Thereafter, circulation is halted, and water and/or molasses feed are introduced at specified locations to displace betaine, sucrose and rest molasses from separate columns in the loop. The xe2x80x9ccirculationxe2x80x9d taught by the ""957 patent is not a true recycle conventional to continuous SMB systems; it merely functions to displace the separation profile to an assigned location in the resin bed. By contrast, the continuous recycle stream, which is essential to a continuous SMB operation, circulates the separation profile continuously through the resin bed.
International Application WO 96/10650 describes a proposed betaine-recovery process which accepts the conventional 92 purity standard as the applicable goal for the sucrose fraction of the process. The WO 96/10650 process contains no suggestion that a sucrose fraction free from significant quantities of betaine or other small organic molecules be collected.
Ordinarily, the chromatography of sucrose-containing mixtures, such as sugar beet molasses, involves variations of elution chromatography. The feed mixture enters a chromatographic configuration of some type, and due to a preferential adsorption, the sucrose product is collected somewhere roughly from the middle to the trailing edge of the developed elution profile. As an elution system is loaded higher and higher with feed mixture, or as eluent is progressively reduced, the bands of separated material broaden and increasingly overlap. As a consequence, separation deteriorates. Inevitably, the efficiency of elution chromatography is limited by mixture loading and elution volume.
It is understood by those skilled in batch chromatography (as opposed to simulated moving bed chromatography) that very high column loading under appropriate conditions provides potentially advantageous specific effects. At large sample load, the components of the mixture to be separated can interfere, and the elution peaks can be modified. A favorable case occurs when the most retained component has the highest saturation capacity. In that case, the most retained component pushes the least retained component ahead of it, and the separation can actually be much better than could be expected from elution mechanisms. Because of this xe2x80x9cdisplacement effect,xe2x80x9d high load separation can actually be superior to separation under elution conditions. When good displacement conditions are obtained, a batch column can be overloaded beyond the level expected from evaluation of data determined with analytical chromatograms of individual components. In batch displacement chromatography, the retained component with high saturation capacity, referred to as the xe2x80x9cdisplacer,xe2x80x9d is most often a molecule chosen for its useful displacement characteristics, and is not necessarily a component of the mixture to be separated.
There remains a need for an improved SMB method for purifying sucrose solutions wherein the sucrose-containing product is not contaminated by cross-over nonsucroses, the betaine and/or invert are recovered almost completely, and the advantages of SMB chromatography are not compromised.
In general terms, this invention constitutes an improvement to a simulated moving bed process for the recovery of a first product fraction predominating in a first product species from a liquid mixture containing that first product species in admixture with a second byproduct species. A feed stock is introduced to a recycle stream circulating through a partitioned bed of resin, raffinate is withdrawn from the recycle stream downstream from the introduction of the feed stock, eluent is introduced to the recycle stream downstream from the withdrawal of raffinate, and extract is withdrawn from the recycle stream downstream from the introduction of eluent, all in usual SMB fashion. The feed stock usually comprises a relatively large amount of the product species and a relatively small amount of the byproduct species.
The improvement of this invention generally comprises establishing a continuous simulated moving bed system in which a feed stock, comprising a relatively large amount of a product species and a relatively small amount of a byproduct species, is fed into a recycle stream circulating through a partitioned bed of resin, the recycle stream being characterized by a separation profile. A raffinate, comprising separated product species and containing other contaminant species, is removed from the recycle stream in the vicinity of the leading edge of the separation profile, downstream from the introduction of feed stock. Eluent is introduced to the recycle stream downstream from the withdrawal of raffinate, and an extract, comprising separated byproduct species, is withdrawn from the recycle stream downstream from the introduction of eluent, in the vicinity of the trailing edge of the separation profile. A significant improvement constitutes accumulating into the recycle stream sufficient displacer species to displace portions of the product species toward the leading edge of the separation profile of the recycle stream. Ideally, the displacer species is the byproduct species.
From another view, in the operation of a continuous simulated moving bed system to separate the components of a feed stock, wherein a resin bed is divided into a series of discrete vessels, each of which functions as a zone within a circulation loop through which is maintained a relatively large-volume continuous recycle stream, and a manifold system connects the vessels and directs in appropriate sequence to each such vessel relatively small-volume streams of feed stock and eluent, respectively, and from each vessel relatively small-volume streams of extract and raffinate, respectively, whereby to develop a circulating inventory of chromatographically separated chemical species characterized by a separation profile including an intermediate region occupied by a commercially valuable phase bounded by a trailing region and a leading region, the invention offers an improvement which comprises operating the system to establish a high inventory of a trailing separated species, whereby to displace a commercially valuable separated species into that leading region, and collecting a raffinate stream from that leading region of the separation profile of the recycle stream.
According to certain practical embodiments of this invention, betaine and/or invert is removed from sucrose solutions via SMB chromatography in near totality prior to purification of the sucrose. These two separations are ideally conducted in coupled SMB systems, with the raffinate produced by the first SMB (SMB A) being processed by a second SMB (SMB B). Independent and different operating parameters (appropriate for each SMB), independent inventory profiles, and independent continuous internal recycle loops are employed for these two respective operations.
The advantageous functions of SMB chromatography are insured by maintaining inventory build-up and continuous internal circulation on each SMB. The betaine and/or invert are recovered in near totality from a first extract fraction. The sucrose product, recovered as an extract from the raffinate fraction resulting from recovery of the first extract fraction, is of substantially higher purity than that produced by conventional operation due to the near total elimination of the crossover nonsucroses (betaine/invert).
In general, this invention may be viewed as an improvement over a simulated moving bed process for the recovery of sucrose from a raw sugar solution in which a feed stock is introduced to a recycle stream circulating through a partitioned bed of resin, raffinate is withdrawn (as a small-volume xe2x80x9csamplexe2x80x9d) from that recycle stream downstream from the introduction of the feed stock, a compensating volume of water is introduced to that recycle stream downstream from the withdrawal of the raffinate, and a balancing xe2x80x9csamplexe2x80x9d amount of sucrose is withdrawn from that recycle stream downstream from the introduction of water. The purity of the sucrose recovered from SMB processing is enhanced in accordance with this invention by establishing first and second chromatographic procedures, the first of which is a continuous SMB, and the second of which can be of any convenient chromatographic configuration, including continuous SMB, sequential SMB or batch configurations. Continuous simulated moving bed configurations are currently preferred for both the first and second chromatographic procedures, and this disclosure makes primary reference to configurations which couple two continuous simulated moving beds, designated for convenience, SMB A and SMB B.
SMB A is operated such that a first feed stock, comprising sucrose and nonsucroses, is fed into a first recycle stream circulating through a first partitioned bed of resin; a first raffinate, comprising sucrose, ash and high molecular weight compounds separated from the feed stock, is removed from the first recycle stream downstream from the introduction of the first feed stock; water is introduced to the first recycle stream downstream from the withdrawal of the first raffinate; and a first extract, comprising nonsucroses separated from the first feed stock, is withdrawn from the first recycle stream downstream from the introduction of water. A second simulated moving bed, SMB B, is operated such that a second feed stock, comprising the first raffinate (from SMB A), is fed into a second recycle stream circulating through a second partitioned bed of resin; a second raffinate, comprising ash and high molecular weight compounds separated from the first raffinate, is removed from the second recycle stream downstream from the introduction of the second feed stock; water is introduced to the second recycle stream downstream from the withdrawal of the second raffinate; and a second extract, comprising sucrose, is withdrawn from the second recycle stream downstream from the introduction of water.
Broadly, the invention can be viewed as a type of xe2x80x9cdisplacement chromatographyxe2x80x9d applied to a simulated moving bed operation. This mechanism results in remarkable and unexpected improved efficiency of separation. According to this invention, forcing a very high steady state inventory of small trailing organic molecules in a simulated moving bed causes a useful xe2x80x9cdisplacement effect.xe2x80x9d Unlike batch displacement chromatography, the displacer is maintained in an equilibrated state, thereby providing a type of xe2x80x9ccontinuous displacement chromatography.xe2x80x9d With sucrose mixtures, such as sugar beet or sugar cane solutions, the displacer is ideally part of the feed mixture, rather than an added component. The practice of the novel continuous displacement chromatography of this invention offers several unique and advantageous characteristics when applied to sucrose solutions. For example:
1. Because displacement rather than elution is key to the separation mechanism, the quantity of eluent used to separate the small organic molecules from the sucrose is very low. Water added to a chromatographic separation process must generally be recovered. Thus, a low requirement for water addition is economically advantageous. Typical molasses chromatographic separation systems use ratios of about 6.0-8.0 water volume for each volume of feed molasses (assuming the feed is 60% dissolved solids). By contrast, the displacement procedure of this invention permits the small trailing organic compounds to be separated almost entirely from the sucrose with water-to-feed ratios of 2.0 or less.
2. The displacement effect results in greater than 90% separation of the trailing small organic molecules, such as betaine, from the sucrose. These compounds are usually difficult to separate from the sucrose in industrial scale operations.
3. The concentration of the small organic molecule fraction is very high compared with the corresponding fraction recovered through elution techniques. Assuming an initial molasses of 60 purity and 60% dissolved solids, elution techniques typically yield betaine-containing fractions of 1% to 5% dissolved solids. With the disclosed displacement method, typical betaine-containing fractions contain about 8% to 15% dissolved solids.
4. The displacement effect forces a very high recovery of the sucrose, typically greater than 99%. It has not previously been recognized that building a large inventory of nonsucrose in a chromatographic separator will improve the sucrose recovery.
5. In conventional industrial sucrose elution chromatography, with strong cation ion exchange resins as the stationary phase, sucrose is a preferentially adsorbed component. As a result, the sucrose is collected somewhere roughly from the middle to the trailing edge of the developed elution profile. In contrast, the displacement procedure of this invention results in sucrose being pushed in exactly the opposite directionxe2x80x94into the area of the leading edge. The key advantage of this contradictory effect is that the sucrose is collected with conventionally easily separated leading edge components such as ash and very high molecular weight compounds. The coupled simulated moving bed aspect of this invention is, therefore, greatly enhanced because the difficult-to-remove small organic nonsucroses are absent following a first SMB so that a subsequent second SMB operating in a conventional manner can recover the sucrose at extremely high purity. Purities of final sucrose fractions from sugar beet molasses have been observed to exceed 97% on dissolved solids. High purity products are generally advantageous, and in this case, the benefits include improved processing characteristics, such as faster crystallization kinetics and lower product odor.
6. The final sucrose fraction is typically subjected to crystallization to obtain the sucrose as a pure product along with a second molasses. The new process of this invention is capable of producing a 96% purity sucrose fraction at 96% recovery so that the overall recovery of product sucrose after the combined operations of chromatographic separation and crystallization increases from the present practical standard of 78% to a new practical standard of about 90%.
7. Simulated moving bed operations require a certain length of time (number of cycles) to equilibrate. With the procedure of this invention, equilibration time can be reduced by adding previously stored or prepared small organic molecule fraction to the feed material or to an appropriate location in the chromatographic separator.